Three-Micron Spectra of Hydrated Asteroids

Over 70 asteroids have been studied in the 3-pm region for hydrated minerals, and these minerals have been found on roughly half of asteroids surveyed [I-61. Recent years have seen hydrated minerals discovered on asteroids of the E and M classes, previously thought to be anhydrous [1,5], and new suggestions for specific minerals on 1 Ceres [7]. With the increasing number of both asteroids and asteroid classes discovered to have hydrated minerals, we can begin to try to identify how many different hydrated minerals are present on asteroid surfaces, what distribution the minerals have within and between classes, and what distribution the minerals have with solar distance. In an effort to accomplish this we have made observations with spectral resolution N 75 of four asteroids known to be hydrated (2 Pallas, 24 Themis, 44 Nysa and 92 Undina) from the IRTF in October, 1994. Preliminary reduction of these data are consistent with previously published data on these asteroids. Figures 1 and 2 show preliminary spectra for 44 Nysa and 92 Undina. Neither one shows evidence of a 3.1 pm feature, although Undina shows a feature at 3.25 pm. Specific identifications of hydrated minerals on asteroid surfaces of many different classes will allow us to better identify both relationships among these classes, and differences within classes. A broad absorption feature from 2.7-3.5 pm is present on bodies containing hydrated minerals, whether silicates or salts (e.g. sulfates, carbonates). Since 1978 these features have been found on at least one asteroid in nearly every spectral class. A feature has been seen within the broad hydration absorption on Ceres, centered at roughly 3.1 pm [6]. This feature was first believed to be water ice, but King et al. [7] supported by Larson et al. [8] suggest that the feature may be due to an ammoniated silicate. The asteroid 2 P d a s does not have a 3.1 pm feature in the data of Jones et al. [I] but has a hydration feature with a substantially different shape than Ceres', indicating that a different mineral (or minerals) is responsible. Because much of the past work on other asteroids in this spectral region was done with very low spectral resolution (AA/A N 15), interpretation of which specific hydrated minerals are present is difficult. Other recent work has suggested that the C, B and G classes (the classes to which 24 Themis, 2 Pallas and 1 Ceres belong) are related along an alteration sequence [9]. Vilas has suggested the "hydrated M7s" or W-class asteroids (like 92 Undina) may also belong on this sequence [lo]. Our observations thus concentrated on these asteroids in order to look for genetic links among them. Our observations covered the spectral region from 2.2-3.5 pm, with points at 2.20, 2.35 and 2.45 pm and every 0.04 pm from 2.90 to 3.50 pm. These data will be presented at the meeting. Future work includes studying other members of the C, E, M, and G classes, and the Themis dynarnical family, to look for variations within classes. References: [I] Jones et al. (1990) Icarus 88, 172-192. [2] Feierberg et al. (1981) Icarus 63, 183-191. [3] Lebofsky L A. (1978) MNRAS 182, 17-21. [4] Lebofsky et al. (1990) Icarvs 83, 12-26. [5] Rivkin et al. (1994) LPSC25, 1135-1136. [6] Lebofsky et al. (1981) Icaws 48,453-459. [7] King et al. (1992) Science 255, 1551-1553. [8] Larson et al. (1994) BAAS 26 1175. [9] Hiroi et al. (1993) Science 261, 1016-1018. [ lo] Vilas (1994) Icarus 111 456-467.